Edge-triggered toggle flip-flop circuit

ABSTRACT

An edge-triggered flip-flop circuit in which a pair of capacitors are alternately charged and discharged to voltages approximating supply rail values and, in combination of with a small number of switches, present high or low impedance paths for input signal transitions of a predetermined polarity to trigger state changes. In an alternative embodiment large switching capacitors are avoided in a circuit that employs a pair of pass-transistor configurations to connect respective capacitors to output terminals of a bistable device. The voltages on the capacitors track the corresponding bistable device output voltages when the input signal is in a given state (illustratively low), and store the value of the corresponding voltage when turned off by the (illustratively high) other state of the input signal. Then, the voltage on the capacitors and the selected input signal transition is used to effectively trigger a transition in the bistable device.

This application is a divisional application Ser. No. 09/407,535, filed on Sep. 29, 1999 now U.S. Pat. No. 6,621,319, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. § 120.

RELATED APPLICATIONS

Subject matter relating to that of the present application is presented in U.S. Patent applications filed on the same day as this application: “Spiking Neuron Circuit” by R. Sarpeshkar; and “Spike-Triggered Asynchronous Finite State Machine,” by R. Herrera and R. Sarpeshkar. These applications are assigned to the assignee of the present application and are hereby incorporated by reference in the present application.

FIELD OF THE INVENTION

The present invention relates to electronic circuits, and, more particularly to bistable circuits for use as digital building block circuits used in electronic systems. Still more particularly, embodiments of the present invention relate to edge-triggered toggle flip-flop circuits.

BACKGROUND OF THE INVENTION

Flip-flop circuits, including edge-triggered toggle flip-flops, are used as building blocks in many digital systems including counters, parity detectors, and registers. In response to each positive/negative edge of an input, usually referred to as the clock, the flip-flop changes its state: It toggles from a ‘0’ to a ‘1’, or from a ‘1’ to a ‘0’.

The prior art has used a number of techniques to avoid constant oscillatory behavior after the input edge has passed. Thus, for example, conventional toggle flip flops are configured so that each edge of a toggle input generates a brief input pulse that is of just the right duration so that the flip flop has sufficient time to change state only once. This strategy proves unreliable, since it relies on a high degree of accuracy in pulse timing. Alternatively, two bistable elements are sometimes configured in a “master-slave” feedback-loop configuration such that one bistable element is written on one phase of the clock signal while the other holds its state and vice versa. This second strategy is reliable but inefficient in its use of circuit area in integrated circuit (IC) chip implementations, since it requires two bistable elements and a number of logic gates for its operation.

SUMMARY OF THE INVENTION

The present invention overcomes limitations of the prior art and achieves a technical advance in providing a novel edge-triggered flip-flop circuit that is reliable in its operation and efficient in its use of IC chip area. Illustrative embodiments of the present invention use only one bistable element and some simple transistor-level logic for its operation.

In a first illustrative embodiment, capacitors are alternately charged and discharged to voltages approximating supply rail values and, in combination with a small number of switches, present high or low impedance paths for input signal transitions of a predetermined polarity (illustratively positive-going), thereby to selectively communicate pulses to switches capable of initiating state transitions in a bistable element.

An alternative embodiment of the present invention provides a flip flop circuit with reduced power requirements that proves useful in a variety of low-power applications. More specifically, potentially high power consumption of large switching capacitors is avoided in a circuit that employs a pair of pass-transistor configurations to operate as switches responding to the input signal (and its complement) to connect respective capacitors to output terminals of a bistable device. In operation, the voltage on the capacitors track the corresponding output voltages when the input signal is in a given state (illustratively low), and store the value of the corresponding voltage when turned off by the (illustratively high) other state of the input signal. Then, the voltage on the capacitors and the selected input signal transition are used to effectively trigger a transition in the bistable device.

BRIEF DESCRIPTION OF THE DRAWING

The above-summarized invention will be understood more completely upon a consideration of the following detailed description read in light of the included drawing, wherein:

FIG. 1 is a circuit diagram of a first illustrative embodiment based on the present inventive teachings.

FIG. 2 shows input-output waveforms useful in understanding the operation of the circuit of FIG. 1.

FIG. 3 shows additional waveforms relating to the circuit of FIG. 1.

FIG. 4 is a circuit diagram of an alternative embodiment of the present invention.

FIG. 5 shows input-output waveforms useful in understanding the operation of the circuit of FIG. 4.

FIG. 6 shows additional waveforms relating to the circuit of FIG. 4.

DETAILED DESCRIPTION

FIG. 1 is a circuit diagram of a first illustrative embodiment based on the present inventive teachings. In FIG. 1 and elsewhere in the present detailed description, transistors M_(i) and capacitors C_(x), for all i and x, are of standard design suitable for manufacture in accordance with a variety of standard processes. Advantageously, CMOS MOSFET designs are employed, but any of PMOS, NMOS, or a combination of these and other types of devices may be employed to advantage in particular contexts. In keeping with standard practice, illustrative p-channel devices (such as M₃) include a “bubble” on the gate, denoting a device that turns on as the gate is made more negative relative to the source. Likewise, the positive power supply is normally positioned at the top of diagrams, with negative voltages at the bottom. (Power supply voltages in circuit diagrams herein will be referred to as V_(DD), with ground being the reference voltage.) So, sources of p-channel devices are at the top, while sources for n-channel devices (such as M₉) are at the bottom. See generally, J. Rabaey, Digital Integrated Circuits, Prentice Hall, New Jersey, 1996; and C. Mead, Analog VLS1 and Neural Sysems, Addison-Wesley, Reading Mass., 1989, especially chapter 3.

Returning to FIG. 1, transistors M₁ and M₃ form an inverter, as do the transistors M₂ and M₄. These two inverters are crosscoupled to form a classic bistable element often referred to as a latch, which latch is arranged to provide outputs Q and Q_(b). Transistors M₅ and M₆ serve to set the state of the latch to ‘1’ (i.e., the voltage of Q very close to V_(DD)) or to ‘0’ (the voltage of Q very close to ground), respectively. Transistors M₅ and M₆ advantageously have a W/L-ratio that is large enough to overwhelm the current from M₃ or M₁. It will be appreciated that transistors M₅ and M₆ compete with each other in setting the latch to a ‘1’ or ‘0’ state; robust setting of the latch is achieved if one of the Q_(s), or Q_(sb) voltages is near V_(DD) while the other voltage is near ground.

If the latch is currently in the ‘1’ state (Q near V_(DD)), transistor M₁₀ is activated, and voltage at Q_(sb) (on the positive side of capacitor C₁) is discharged to ground; further, because M₁₀ is strongly activated, any capacitive coupling of the input V_(in) from C₁ is weak. In contrast, in the ‘1’ state, the M₁₁ transistor is inactive because node Q_(b) is near ground, the Q_(s) node is in a high-impedance state, and any capacitive coupling of the input V_(in) from C₂ (chosen to be near C₁ in capacitance) is strong. Thus, in the ‘1’ state, a positive-going edge from the input will activate M₆ and reset the latch to ‘0’.

Similarly, when the latch of FIG. 1 is in the ‘0’ state (voltage at Q_(b) is near V_(DD)), a positive edge at the input V_(in) will activate M₅ and reset the latch to ‘1’. Thus, the state of the latch toggles between a ‘1’ and ‘0’ in response to a sequence of positive-going edges at V_(in). It should be noted that as soon as the latch changes state, it inactivates whichever of the Q_(s) or Q_(sb) nodes caused it to change state, thus resetting both nodes to ground after a state change has occurred. See FIGS. 3A-E.

The transistors M₉ and M₁₂ are advantageously weakly turned on with a constant voltage V_(lk) so as to prevent large negative excursions in the Q_(sb) or Q_(s) voltages from occurring whenever a negative edge is coupled from the input to a high-impedance node. As shown in FIGS. 3D and 3E, negative edges in the input signal, V_(in), have no effect in the circuit apart from causing small negative transients at the Q_(s) or Q_(sb) nodes. The transistors M₈ and M₇ perform an asynchronous clear function and reset the state of the flip flop to ‘0’ whenever the clear input is near V_(DD). When the clear input is active, M₇ resets Q to zero, which causes Q_(b) to go high and reset Q_(s) to zero; the transistor M₈ resets Q_(sb) to zero.

FIG. 2 shows waveforms occurring during operation of the circuit of FIG. 1 for the case of devices designed for a 0.5 μm manufacturing process with V_(DD)=3.3V, V_(lk)=0.9V, and C₁=C₂=0.1 pF. Note that, on every positive edge of the input V_(in), the output Q changes state as expected for a toggle flip-flop. The other output Q_(b) is a faithful inverted replica of Q. FIGS. 3A-E display waveforms as in FIG. 2 but, in addition, depict the behavior of the Q_(s) and Q_(sb) nodes. Note that on each positive edge, only one of the Q_(s) or Q_(sb) nodes is activated. On each negative edge, the high-impedance node suffers a larger negative excursion than does its low-impedance counterpart; nevertheless, the negative excursion is not large enough to prevent state changes of the circuit from arising when the subsequent positive edge arrives.

A potential disadvantage of the circuit of FIG. 1 for some design contexts arises when coupling capacitors C₁ and C₂ assume undesirably high values when chosen to avoid undesired attenuation of the input signal by parasitic capacitances at the Q_(s) and Q_(sb) nodes. Any such undesirably high values of C₁ and C₂ may, in turn, lead to unnecessary switching power dissipation.

FIG. 4 shows an alternative embodiment of the present invention having relatively low power dissipation because of smaller capacitances employed in its operation. This reduction in power dissipation is usually achieved using a higher transistor count and circuit area relative to the circuit of FIG. 1.

In the circuit of FIG. 4, there are no floating capacitors (all capacitors have one of their terminals tied to a D.C. voltage, typically, ground). In typical implementations, the capacitors are not explicitly implemented, but arise as parasitic capacitances. For particular very low frequency operation of the toggle flip-flop of FIG. 4, explicit capacitors may be employed, as discussed below.

In FIG. 4 transistors M₄₁-M₄₂ and M₄₃-M₄₄ form a latch as in the circuit of FIG. 1. The parallel connection of transistors M₄₉ and M₄₁₀, or the parallel connection of transistors M₄₁₁ and M₄₁₂ form a pass-transistor configuration. See generally, for example, J. M. Rabaey, Digital Integrated Circuits, Prentice-Hall, 1996, pp. 410-422. The pass-transistor configuration is gated by the voltage V_(in) and its complementary counterpart V_(inb) to act as a conducting switch when V_(in) is low. The complementary signal V_(inb) may be readily obtained by passing V_(in) through a simple CMOS inverter (not shown), as will readily be appreciated by those skilled in the art.

When V_(in) is high, the pass-transistor switch is turned off. Thus, the voltage at Q_(sb) (the voltage on C₄₁) tracks the Q_(b) voltage when V_(in) is low but holds its previous value when V_(in) is high. Similarly, the Q_(s) voltage on C₄₂ tracks the Q voltage when V_(in) is low but holds its previous value when V_(in) is high. Thus, during the active high phase of the input, only one of the Q_(s) or Q_(sb) voltages is high according to whether Q or Q_(b) was high during V_(in)'s preceding inactive low phase. Consequently, when V_(in) goes high, only one of the M₄₅-M₄₆ or M₄₇-M₄₈ arms will conduct current and cause the latch to change its state from ‘0’ to ‘1’ or from ‘1’ to ‘0’. The positive edge of the V_(in) input, thus causes the latch to toggle its state. The Clr input on transistor M₄₁₃ resets the state of the latch to ‘0’.

The circuit of FIG. 4 thus operates in a manner similar to conventional master-slave toggle flip-flops, but with the important difference that functions of a slave are performed by the implicit or explicit capacitors C₄₁ and C₄₂ rather than by a second bistable element.

To ensure proper operation of the circuit for arbitrarily slow periods of the input, the capacitors C₄₁ and C₄₂ will be chosen large enough that their hold time is significantly greater (5-10 times) than the switching time of the latch. In that case, even if there are slow leakage currents to ground that cause degradation of the voltage values held at nodes Q_(s) or Q_(sb), such degradation does not adversely affect the operation of the circuit. This robustness of operation arises because the quick switching time of the latch allows it to change its state in a time that is much less than the time in which the Q_(s) and Q_(sb) voltages degrade. Once the latch has changed state, its positive feedback enables it to hold its new state even if the Q_(s) and Q_(sb) inputs have leaked their charge to ground.

To ensure that all leakage is to ground and not to V_(DD), optional transistors M₄₁₄ and M₄₁₅ may be added across respective capacitors C₄₁ and C₄₂ to ground. Optional transistors M₄₁₄ and M₄₁₅ allow capacitor leakage to be explicitly controlled via subthreshold biasing of the (optional) V_(lk) voltage, as in the circuit of FIG. 1. Since very low frequency operation is usually not important in most digital circuits, C₄₁ and C₄₂ are typically implicit capacitors, and transistors M₄₁₄ and M₄₁₅ (and voltage V_(lk)) are typically not required.

FIG. 5 shows shows typical waveforms for the circuit of FIG. 4 using a standard 0.5 μm process with V_(DD)=3.3V, C₄₁=C₄₂=0.01 pF, and the leak transistors M₄₁₄ and M₄₁₅ being absent. Note that, on every positive edge of the input V_(in), the output Q changes state as expected of a toggle flipflop. The other output Q_(b) is a faithful inverted replica of Q. FIG. 6 reveals the same waveforms of FIG. 5 and, in addition, also depicts the behavior of the Q_(s) and Q_(sb) nodes. Note that, apart from minor glitches, both Q_(s) and Q_(sb) hold their values during the active high phase of V_(in) and track Q and Q_(b) respectively during the inactive low phase of V_(in).

The glitches on Q and Q_(b) on the negative edges of V_(in) arise because Q_(s) and Q_(sb) temporarily load the latch inverters as they transition from their held state to the current state. The glitches on Q_(s) and Q_(sb) during the high phase of V_(in) arise because of capacitive coupling from the drains of M₄₆ and M₄₈ to their gates as V_(in) rises and pulls these drains high via M₄₅ and M₄₇ respectively; the coupling is larger when Q_(s) or Q_(sb) are at high values because the gate-to-drain capacitance of M₄₆ or M₄₈ is larger when their gates are high.

It will be understood by those skilled in the art that device types, manufacturing process, polarities and parameter values used in the above-described embodiments of the present invention are merely illustrative. Other embodiments and variations within the spirit of the present invention will be apparent to those skilled in the art in light of the present inventive teachings.

Those skilled in the art will recognize that complementary circuit designs based on the present inventive teachings allow operation of circuits based on negative transitions. 

1. A toggle flip-flop circuit comprising a bistable element having first and second output terminals exhibiting respective complementary first and second binary output voltages, and triggering means for initiating a change of state for said bistable element from a present output state to a complementary output state, said change of state causing both said first and second terminals to change binary output voltages, said triggering means comprising a first capacitor having a first terminal connected to a reference voltage and a second terminal connected through a first switch to said first output of said bistable element, a second capacitor having a first terminal connected to said reference voltage and a second terminal connected through a second switch to said second output of said bistable element, said first and second switches controlled by a binary input signal such that when said binary input signal is in a first state said switches connect said second terminal of respective ones of said capacitors to respective outputs of said bistable element when said binary input signal is in a second state said switches disconnect said capacitors from said respective outputs of said bistable element, means responsive to said voltages at said capacitors for switching the binary state of said bistable element, and means for inhibiting transitions in said input signal from affecting said means responsive to voltages at said capacitors for switching the binary state of said bistable element.
 2. The toggle flip-flop circuit of claim 1 wherein said bistable element comprises first and second inverters, each inverter having an input terminal and an output terminal, output terminals of each inverter being connected to the input terminal of the other inverter, output terminals of each of said inverters corresponding to respective ones of said output terminals of said bistable element.
 3. The toggle flip-flop circuit of claim 1 wherein said means responsive to voltages at said capacitors for switching the binary state of said bistable element comprises means for connecting the output terminal of one of said inverters to said reference voltage.
 4. The toggle flip-flop circuit of claim 1 wherein said switches comprise pass transistor configurations responsive to said input signal and a complementary of said input signal.
 5. The toggle flip-flop circuit of claim 1, wherein said means for inhibiting comprises a first transistor connected in parallel across said first capacitor and a second transistor connected in parallel across said second capacitor, said first and second transistors operating in response to a constant applied voltage.
 6. The toggle flip-flop circuit of claim 1 wherein said reference voltage is ground.
 7. The toggle flip-flop circuit of claim 1 further comprising means for resetting said bistable element to an initial state.
 8. The toggle flip-flop circuit of claim 7 wherein said means for resetting comprises a switch selectively connecting at least one of said output terminals to said reference voltage. 